When a sharply focused electron beam is made to impinge on a specimen, reflected electrons and secondary electrons are emitted from the specimen. The intensities of these emitted electrons differ, depending on the topography of the specimen surface and also on the elements constituting the specimen. Accordingly, the reflected electron image can be separated into a topographical image and a compositional image of the specimen by using two detectors which are arranged symmetrically with respect to the optical axis to detect reflected electrons, for example. A topographical signal is derived from the difference between the output signals from the two detectors while the sum of the output signals is a signal representing the elements of the specimen. This technique is described in U.S. Pat. No. 3,329,813.
The generally accepted opinion is that the difference between the output signals from the detectors arranged symmetrically with respect to the optical axis depends only on the topography of the specimen surface. In practice, however, the intensity of released electrons varies among atomic numbers. Therefore, the difference signal does not accurately represent the topography.
It is customary to coat the specimen surface with a substance of a single atomic number, e.g., a single metal such as gold, prior to measurement to prevent the intensity of emitted electrons from being affected by the elements of the specimen. However, the amount of the coating substance deviates from the intended amount since the substance is an aggregation of particles. Especially at high magnifications, it is difficult to accurately image the topography of the specimen surface.
After a topographical signal is produced from reflection electron detectors, it is integrated by an integrator. In order to know the absolute value of the height of the output signal from the integrator, the conditions of the incident electron beam must be established. However, the amplitude of the output signal from each detector varies normally when the accelerating voltage of the electron beam is varied or when the bias voltage applied inside the electron gun is varied. Also, the amplitude changes with drift. Furthermore, the amplitude varies according to the setting of the saturation point of the filament. In addition, the amplitude is varied when the excitation current flowing through the condenser lens is changed. In these cases, the amplitude of the signal representing the intensity of reflected electrons varies and, therefore, it is impossible to accurately maintain the value obtained by the measurement. Hence, an accurate quantitative measurement cannot be made unless the signal intensity is corrected whenever the dosage of the incident electron beam is varied. For this reason, where a quantitative measurement of, for example, volume ratio taken in the direction of height or in three dimensions is needed, a change in the dose of the incident electron beam directly gives rise to an error.